Ink composition for ink-jet recording and ink set comprising the same

ABSTRACT

An object of the present invention is to provide an ink composition for ink jet recording, which can produce high-quality prints and has excellent ejection stability, and to provide an ink set using the same. The ink composition for ink jet recording according to the present invention is characterized by comprising at least a compound represented by formula (I)  
                 
 
     wherein  
     EO represents an ethyleneoxy group;  
     PO represents a propyleneoxy group;  
     m1, m2, m3, n1, n2, and n3 each are independently 0 (zero) or a natural number of not less than 1;  
     EO and PO may be arranged, regardless of order in the parentheses [ ], randomly or as blocks joined together; and m1+m2+m3+n1+n2+n3 is in the range of 0.5 to 10 in terms of number average of a mixture of compounds represented by formula (I), and a colorant, provided that when, in formula (I), all of n1, n2, and n3 are zero, the ink composition further comprises glycerin.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an ink composition for ink jetrecording, and an ink set comprising the same.

[0003] 2. Background Art

[0004] Ink jet recording is a printing method wherein droplets of an inkcomposition are ejected and deposited on recording media, such as paper,to print, for example, letters or figures. The feature of the ink jetrecording method is that images having a combination of high resolutionwith high quality can be printed at a high speed by means of arelatively inexpensive apparatus.

[0005] Examples of ink jet recording methods which have been put topractical use include: a method wherein an electric signal is convertedto a mechanical signal using an electrostrictive element tointermittently eject an ink reservoired in a nozzle head section,thereby recording letters or symbols on the surface of a recordingmedium; and a method wherein an ink, reservoired in a nozzle headsection, in its portion very close to the ejection portion is rapidlyheated to create a bubble and the ink is intermittently ejected byvolume expansion created by the bubble to record letters or symbols onthe surface of a recording medium.

[0006] Various properties required of ink compositions used in the aboveink jet recording are such that the drying property of the print isgood, no significant feathering is created in printed images, uniformprinting can be realized on the surface of conventional recording media,and, in the case of multi-color printing, mixing between adjacent colorsdoes not occur. Further, what is important to ink compositions is toensure satisfactory ejection stability of ink at actual servicetemperatures of ink jet recording apparatuses.

[0007] To ensure these properties, various studies have hitherto beenmade on additives to ink compositions.

[0008] For example, they have proposed the use of an additive forlowering the surface tension of the ink composition to increase thepenetration speed of the ink composition into paper and thus to reducefeathering or bleeding. More specifically, the use of diethylene glycolmonobutyl ether (U.S. Pat. No. 5,156,675) or Surfynol 465 (manufacturedby Air Products and Chemicals Inc., U.S.A.) (U.S. Pat. No. 5,183,502) asan acetylene glycol surfactant or the combined use of diethylene glycolmonobutyl ether and Surfynol 465 (U.S. Pat. No. 5,196,056) has beenproposed. Further, an ink composition has been proposed which contains,as a wetting agent, polyglycerin (Japanese Patent Laid-Open No.152170/1991), polyglycerin with an ethyleneoxy group added thereto(Japanese Patent Laid-Open No. 328644/1997), or glycerin with anethyleneoxy group added thereto (Japanese Patent Laid-Open No.18465/1992).

[0009] For these conventional ink compositions, however, there is stillroom for improvement. Specifically in the case of printing on plainpaper, for example, mere addition of an adduct of ethyleneoxy groupswith glycerin to ink compositions sometimes causes prolongation ofdrying time due to high surface tension. Therefore, when printing iscontinuously carried out on a plurality of sheets, there is fear thatsatisfactory drying time cannot be ensured. Thus, immediately afterprinting, prints cannot be put on top of one another. This isdisadvantageous in high-speed printing. Further, this ink compositioncannot in some cases produce high-quality prints.

[0010] An ink jet recording method using an electrostrictive element isadvantageous in that printing can be carried out without thermal damageto the ink composition. Further, a recording head using anelectrostrictive element has a great advantage such that the head caneject ink droplets several hundred millions of times and thus can bepermanently used. In ink using a relatively large amount of solidmatter, such as colorants, when the ink is not ejected for a long periodof time, the ink dries on the front face of the nozzle in the recordinghead and thus is thickened, often leading to print disorder. Bycontrast, according to the head using an electrostrictive element, thisproblem can be overcome by delicately moving the ink at the front faceof the nozzle on such a level that does not eject the ink from thenozzle, thereby stirring the ink. Therefore, the ink jet recordingmethod using the electrostrictive element is advantageous in recordingusing an ink composition containing a high concentration of a colorant,such as a pigment.

[0011] Japanese Patent Laid-Open No. 157698/1995 discloses examples ofink compositions to which a 1,2-alkylene glycol has been added to reducebleeding of the dye. This publication, however, does not disclosepigment-based ink compositions. In general, as compared with dyes,pigments are advantageously superior in waterfastness, lightfastness,and weatherfastness, and, at the same time, is less likely to causecolor mixing. Further, so far as the present inventors know, mere use ofthe 1,2-alkylene glycol in ink compositions is likely to cause cloggingin a head using an electrostrictive element.

[0012] Japanese Patent Laid-Open No. 18465/1992 discloses an inkcomposition containing some of the compounds represented by formula (I)which will be described later. This ink composition is used in an inkjet recording method wherein ink is ejected from an orifice of therecording head through the action of thermal energy to performrecording. In this case, the compound represented by formula (I) is usedfor improving the ejection stability of the ink in this recordingmethod. This publication, however, does not disclose the combined use ofthe compound represented by formula (I) and glycerin.

SUMMARY OF THE INVENTION

[0013] The present inventors have now found that an ink compositioncomprising at least a compound having a specific structure and acolorant has excellent ejection stability and the use of this compoundpermits the viscosity of the ink composition to be easily regulated to adesired value. The present inventors have further found that, in an inkset comprising at least two ink compositions of the type describedabove, the ink set can yield high-quality prints by modifying theviscosities of the ink compositions to respective predeterminedviscosity ranges. The present invention has been made based on suchfinding.

[0014] Accordingly, it is an object of the present invention to providean ink composition for ink jet recording, which has excellent ejectionstability, and an ink set for ink jet recording which comprises theabove ink composition and can offer excellent print quality and ejectionstability.

[0015] According to one aspect of the present invention, there isprovided an ink composition for ink jet recording, comprising at least acompound represented by formula (I)

[0016] wherein

[0017] EO represents an ethyleneoxy group;

[0018] PO represents a propyleneoxy group;

[0019] m1, m2, m3, n1, n2, and n3 each are independently 0 (zero) or anatural number of not less than 1;

[0020] EO and PO may be arranged, regardless of order in the parentheses[ ], randomly or as blocks joined together; and

[0021] m1+m2+m3+n1+n2+n3 is in the range of 0.5 to 10 in terms of numberaverage of a mixture of compounds represented by formula (I), and acolorant, provided that

[0022] when, in formula (I), all of n1, n2, and n3 are zero, said inkcomposition further comprises glycerin.

[0023] According to another aspect of the present invention, there isprovided an ink set for ink jet recording, comprising at least two inkcompositions of the type described above, characterized in that the inkset satisfies a relationship represented by formula (a) at least at 20°C.:

((μ max−μ min)/μ max)×100≦5 (%)  (a)

[0024] wherein μ max represents the maximum viscosity value in the inkcompositions of the ink set; and μ min represents the minimum viscosityvalue in the ink compositions of the ink set.

[0025] According to the ink composition and the ink set of the presentinvention, prints with a lowered level of feathering or bleeding can berealized on plain paper, for example, on recycled paper, and, thus,prints having higher quality can be produced. Further, this ink set,even when used in recording heads using an electrostrictive element, isless likely to cause clogging, and, thus, can realize excellent ejectionstability.

DETAILED DESCRIPTION OF THE INVENTION Ink Composition

[0026] The ink composition according to the present invention is an inkcomposition for ink jet recording. Here the ink jet recording methodgenerally refers to a method wherein ink droplets are ejected anddeposited onto a recording medium to perform printing. In the presentinvention, ink jet recording methods include a method wherein dropletsof an ink composition are ejected using an electrostrictive element andare deposited onto a recording medium to perform printing, and a methodwherein bubbles of an ink composition are generated by partial heatingand are ejected to perform printing.

[0027] The ink composition of the present invention is preferably usedin an ink jet recording method wherein ink droplets are ejected by meansof an electrostrictive element and are deposited onto a recording mediumto perform printing.

[0028] The ink composition for ink jet recording according to thepresent invention basically comprises at least a compound represented byformula (I) and a colorant. In this case, in formula (I), EO representsan ethyleneoxy group, PO represents a propyleneoxy group, m1, m2, m3,n1, n2, and n3 each are independently 0 (zero) or a natural number ofnot less than 1, and EO and PO may be arranged, regardless of order inthe parentheses of formula (I), randomly or as blocks joined together.When m1+m2+m3+n1+n2+n3 is expressed in terms of number average of amixture of compounds represented by formula (I), m1+m2+m3+n1+n2+n3 is inthe range of 0.5 to 10, preferably 2 to 8, more preferably 3 to 6. Whenm1+m2+m3+n1+n2+n3 is not less than 0.5, the print quality can beimproved, while when this value is not more than 10, the viscosity ofthe ink composition is advantageously on a level suitable for use in inkjet recording.

[0029] In this connection, it should be noted that in formula (I), whenall of n1, n2, and n3 are zero, the ink composition for ink jetrecording according to the present invention further comprises at leastglycerin in addition to the compound represented by formula (I) and thecolorant.

[0030] The ink composition according to the present invention hasexcellent ejection stability. When the ejection stability of the inkcomposition is good, there is no variation in frequency characteristicsof ink. This can eliminate the need to separately provide mechanisms forregulating ink ejection respectively for inks in the recording head,and, thus, can realize the control of ink ejection through a singlemechanism.

[0031] According to one preferred embodiment of the present invention,in formula (I), even when none of n1, n2, and n3 are zero, the inkcomposition for ink jet recording may further comprise glycerin.

[0032] In the present invention, commercially available products as suchmay be used as the compound represented by formula (I), oralternatively, the commercially available products may be utilized afterproper modification. Further, if necessary, the compound represented byformula (I) may be synthesized using glycerin or the like as a startingcompound. Accordingly, processes usable for the production of thecompound represented by formula (I) include, for example, a productionprocess wherein glycerin is used as a starting compound and target molaramounts of ethylene oxide and propylene oxide are added to glycerin inan atmosphere of an alkali or the like, and a production process whereina glycol compound, such as ethylene glycol, diethylene glycol, orpropylene glycol, is added with dehydration to glycerin.

[0033] In the present invention, the compound represented by formula (I)may be a single compound composed of only one compound selected from thegroup consisting of the compounds represented by formula (I), oralternatively may be a mixture of two or more compounds selected fromthe group consisting of the compounds represented by formula (I). Whenthe single compound selected from among the compounds represented byformula (I) is used, if necessary, the single compound may be separatedfrom a mixture of two or more compounds among the compounds representedby formula (I) by applying a conventional process, such as distillation.

[0034] In the present invention, the compound represented by formula (I)is preferably a mixture of two or more compounds selected from the groupconsisting of the compounds represented by formula (I). The mixture isadvantageous in that, when the ink composition according to the presentinvention is used in a head provided with an electrostrictive element,the anticlogging property is good and better print quality can berealized.

[0035] According to a preferred embodiment of the present invention, thecompound represented by formula (I) is not more than 1000, morepreferably not more than 800, still more preferably 600 to 1000, interms of the average molecular weight. When the average molecular weightis not more than 1000, the viscosity of the ink composition can bebrought to a suitable range for use and, at the same time, prints havinghigher quality can be advantageously realized.

[0036] In the ink composition according to the present invention, theamount of the compound represented by formula (I) added is preferably0.1 to 30% by weight, more preferably 1 to 20% by weight, based on theamount of the whole ink composition.

[0037] According to a further preferred embodiment of the presentinvention, the surface tension of the ink composition for ink jetrecording according to the present invention is not more than 40 mN/m,more preferably 28 to 35 mN/m or less, still more preferably 29 to 33mN/m or less. When the surface tension of the ink composition is notmore than 40 mN/m, the ink composition easily penetrates into ordinarypaper, such as plain paper, and dries in a shortened time. Therefore, anunfavorable phenomenon called “offset” is less likely to take placewherein, in continuous printing, traces of previous print are left onpaper which has been placed on the previous prints. Therefore, this isadvantageous when high-speed printing is contemplated.

[0038] According to another preferred embodiment of the presentinvention, in order to bring the surface tension of the ink compositionto not more than 40 mN/m, the ink composition further contains an1,2-alkylene glycol wherein the alkylene group portion may be branched.In this case, when the content of the optionally branched 1,2-alkyleneglycol in the ink composition according to the present invention is notless than 3% by weight based on the amount of the whole ink composition,the content of the compound represented by formula (I) is preferably atleast 2% by weight, more preferably 3 to 20% by weight.

[0039] Further, in the 1,2-alkylene glycol, the alkylene group portionpreferably has 4 to 10 carbon atoms, more preferably 4 to 6. Morepreferably, the 1,2-alkylene glycol is 1,2-pentanediol, 1,2-hexanediol,or a mixture of 1,2-pentanediol with 1,2-hexanediol. The ink compositionaccording to the present invention preferably contains 1,2-pentanediolin an amount of 0.5 to 20% by weight, more preferably 5 to 10% byweight, based on the ink composition, or preferably contains1,2-hexanediol in an amount of 0.3 to 15% by weight, more preferably 1to 10% by weight, based on the ink composition, or more preferablycontains a mixture of these alkylene glycols.

[0040] The ink composition according to the present invention preferablyfurther contains (di)propylene glycol monobutyl ether in addition to the1,2-alkylene glycol. The surface tension of the ink composition may bebrought to not more than 40 mN/m by this constitution. Further,according to the ink composition of the present invention, the additionof the (di)propylene glycol monobutyl ether can improve the penetrationof the ink composition.

[0041] According to the ink composition of the present invention, theamount of (di)propylene glycol monobutyl ether added is preferably 0 to10% by weight, more preferably 0.5 to 5% by weight, based on the amountof the whole ink composition. When the amount of the (di)propyleneglycol monobutyl ether added is not more than 10% by weight, a preferredpenetration level of the ink composition can be advantageously ensured.Further, according to the ink composition of the present invention, theweight ratio of the 1,2-alkylene glycol to the (di)propylene glycolmonobutyl ether is more preferably 1:0 to 1:10, still more preferably1:0.5 to 1:5.

[0042] The ink composition according to the present invention preferablyfurther contains an acetylene glycol surfactant in addition to the1,2-alkylene glycol. The surface tension of the ink composition may bebrought to not more than 40 mN/m by this constitution. Further, theaddition of the acetylene glycol surfactant can further improve printquality.

[0043] In the ink composition according to the present invention, theamount of the acetylene glycol surfactant added is preferably 0 to 5% byweight, more preferably 0.1 to 2% by weight, based on the amount of thewhole ink composition. When the amount of the acetylene glycolsurfactant added is not more than 5% by weight, the print quality can beadvantageously improved without undergoing the influence of an increasein viscosity of the ink composition.

[0044] In the ink composition according to the present invention, theweight ratio of the 1,2-alkylene glycol to the acetylene glycolsurfactant is more preferably 1:0 to 1:3, still more preferably 1:0.1 to1:1. The addition of the acetylene glycol surfactant in this ratio rangeis preferred from the viewpoint of ensuring excellent print quality.

[0045] Preferred acetylene glycol surfactants usable in the presentinvention include, for example, Surfynol series manufactured by AirProducts and Chemicals Inc.

[0046] The ink composition according to the present invention preferablyfurther contains di(tri)ethylene glycol monobutyl ether in addition tothe 1,2-alkylene glycol. The surface tension of the ink composition maybe brought to not more than 40 mN/m by this constitution. Further, theaddition of di(tri)ethylene glycol monobutyl ether can further improveprint quality. Furthermore, the water solubility of the ink compositioncan be ensured.

[0047] In the ink composition according to the present invention, theamount of di(tri)ethylene glycol monobutyl ether added is preferably 0to 20% by weight, more preferably 0.5 to 10% by weight, based on theamount of the whole ink composition.

[0048] In the ink composition according to the present invention, theweight ratio of the 1,2-alkylene glycol to the di(tri)ethylene glycolmonobutyl ether is more preferably 1:0 to 1:10, still more preferably1:0.1 to 1:5.

[0049] In the ink composition according to the present invention, thecolorant may be properly selected from water-soluble dyes or pigments.The water-soluble dye and the pigment may be optionally properly used incombination.

[0050] According to the present invention, various dyes commonly used inink jet recording, such as direct dyes, acid dyes, foodstuff dyes, basicdyes, reactive dyes, disperse dyes, vat dyes, and soluble vat dyes, maybe used as the water-soluble dye.

[0051] The pigment is not particularly limited, and, for example,various pigments, such as inorganic pigments or organic pigments, may beused. Therefore, according to the present invention, the pigment may beone which is dispersible in an ink composition with the aid of adispersant or a surfactant. The pigment, however, is preferably asurface treated pigment which has a dispersing group on its surface andis dispersible in an aqueous solvent without any dispersant. Here thedispersing group refers to a group which renders a pigment dispersiblein an aqueous solvent solely, that is, without the addition of anydispersant. Specific examples of dispersing groups include carboxyl,carbonyl, hydroxyl, and sulfonic acid groups.

[0052] The introduction of the dispersing group into the surface of thepigment may be carried out by a suitable method according to the type ofthe dispersing group. For example, carboxyl or hydroxyl groups arepreferably introduced by oxidation. Here the oxidation refers to amethod wherein the pigment is oxidized in a liquid phase or gaseousphase with an oxidizing agent, for example, ozone, nitric acid, hydrogenperoxide, hypohalous acid, nitrogen oxide, or fluorine gas. Plasmatreatment of the surface of the pigment also can introduce carboxyl orhydroxyl groups into the surface of the pigment. On the other hand, whensulfonic acid groups are introduced as the dispersing group, thesulfonic acid groups can be introduced by sulfonation.

[0053] The surface treated pigment is also stable against glycol ethers,for example, (di)propylene glycol monobutyl ether and di(tri)ethyleneglycol monobutyl ether which are used as a surface tension modifier.This is advantageous over the dispersion of the pigment with the aid ofa dispersant in that the ink composition can withstand severerconditions and, at the same time, can be stably used even underhigh-temperature or low-temperature conditions.

[0054] In the present invention, the content of the surface treatedpigment is preferably 1 to 15% by weight, more preferably 3 to 10% byweight, based on the ink composition.

[0055] Further, the ink composition according to the present inventionmay properly contain optional ingredients, such as nozzle cloggingpreventives, preservatives, antimolds, antioxidants, electricconductivity adjustors, pH adjustors, viscosity modifiers, other surfacetension modifiers, and oxygen absorbers. These optional ingredients maybe used solely or as a mixture of a plurality of optional ingredientsfalling within the same category or a plurality of optional ingredientsselected from different categories.

[0056] For example, water-soluble glycols or saccharides may be used asnozzle clogging preventives for preventing the ink composition frombeing dried at the front face of nozzles in the recording head.

[0057] Examples of water-soluble glycols usable herein include ethyleneglycol, diethylene glycol, triethylene glycol, propylene glycol,dipropylene glycol, tripropylene glycol, polyethylene glycol having amolecular weight of not more than 2000, 1,3-propylene glycol,isopropylene glycol, isobutylene glycol, 1,4-butanediol, 1,3-butanediol,1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol,1,2,6-hexanetriol, 1,8-octanediol, 1,2-octanediol, glycerin,mesoerythritol, and pentaerythritol.

[0058] Saccharides usable herein include monosaccharides andpolysaccharides. More specific examples thereof include glucose,mannose, fructose, ribose, xylose, arabinose, lactose, galactose,aldonic acid, glucitose, maltose, cellobiose, sucrose, trehalose,maltotriose, alginic acid and salts thereof, cyclodextrins, andcelluloses.

[0059] Examples of compounds which are properly usable in the inkcomposition according to the present invention include glycol ethers,which are compatible with water and have low solubility in watercontained in the ink composition, and compounds which can improve thesolubility of the ink ingredients and, in addition, can improve thepenetration into recording media, for example, paper, or can be used forpreventing clogging of the nozzles. Specific examples of such compoundsinclude: alkyl alcohols having 1 to 4 carbon atoms; glycol ethers, suchas ethylene glycol monomethyl ether, ethylene glycol monoethyl ether,ethylene glycol monobutyl ether, ethylene glycol monomethyl etheracetate, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol mono-n-propyl ether, ethylene glycolmono-iso-propyl ether, diethylene glycol mono-iso-propyl ether, ethyleneglycol mono-n-butyl ether, diethylene glycol mono-n-butyl ether,triethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butylether, diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol,propylene glycol monomethyl ether, propylene glycol monoethyl ether,propylene glycol mono-t-butyl ether, propylene glycol mono-n-propylether, propylene glycol mono-iso-propyl ether, dipropylene glycolmonomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycolmono-n-propyl ether, dipropylene glycol mono-iso-propyl ether, propyleneglycol mono-n-butyl ether, and dipropylene glycol mono-n-butyl ether;formamide; acetamide; dimethyl sulfoxide; sorbit; sorbitan; acetin;diacetin; triacetin; and sulfolane.

[0060] The ink composition according to the present invention mayfurther comprise other surfactant from the viewpoint of regulating thepenetration of the ink composition into recording media, or regulatingthe surface tension of the ink composition. The surfactant is preferablyhighly compatible with the ink composition according to the presentinvention. Further, the surfactant preferably has high penetration andis stable. Such surfactants include, for example, amphoteric surfactantsand nonionic surfactants.

[0061] Specific examples of amphoteric surfactants includelauryldimethylaminoacetic acid betaine,2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine, coconut oilfatty acid amide propyldimethylaminoacetic acid betaine,polyoctylpolyaminoethylglycine, and imidazoline derivatives.

[0062] Specific examples of nonionic surfactants usable herein includeether surfactants, such as polyoxyethylene nonylphenyl ether,polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether,polyoxyethylene alkylallyl ether, polyoxyethylene oleyl ether,polyoxyethylene lauryl ether, polyoxyethylene alkyl ether, andpolyoxyalkylene alkyl ether, polyoxyethyleneoleic acid, estersurfactants, such as polyoxyethyleneoleic ester,polyoxyethylenedistearic ester, sorbitan laurate, sorbitan monostearate,sorbitan monooleate, sorbitan sesquioleate, polyoxyethylene monooleate,and polyoxyethylene stearate, and fluorosurfactants, such as fluoroalkylesters and salts of perfluoroalkylcarboxylic acid.

[0063] For example, sodium benzoate, pentachlorophenol sodium,2-pyridinethiol-1-oxide sodium, sodium sorbate, sodium dehydroacetate,and 1,2-dibenzothiazolin-3-one (Proxel CRL, Proxel BDN, Proxel GXL,Proxel XL-2, and Proxel TN, manufactured by ICI) may be used aspreservatives and antimolds.

[0064] pH adjustors, solubilizers, or antioxidants usable hereininclude: amines, for example, alkanolamines, such as diethanolamine,triethanolamine, and propanolamine, alkylalkanolamines, such asmethyldiethanolamine, dimethylethanolamine, ethyldiethanolamine, anddiethylethanolamine, and morpholine, and modification products thereof;inorganic salts, such as potassium hydroxide, sodium hydroxide, andlithium hydroxide; ammonium hydroxide; quaternary ammonium hydroxide,such as tetramethylammonium; salts of carbonic acid, such as potassiumcarbonate, sodium carbonate, and lithium carbonate; salts of phosphoricacid, such as potassium phosphate, sodium phosphate, and lithiumphosphate; N-methyl-2-pyrrolidone; urea compounds, such as urea,thiourea, and tetramethylurea; allophanates, such as allophanate andmethyl allophanate; biurets, such as biuret, dimethylbiuret, andtetramethylbiuret; and L-ascorbic acid and salts thereof.

[0065] Viscosity modifiers include rosins, alginic acids, polyvinylalcohol, hydroxypropylcellulose, carboxymethylcellulose,hydroxyethylcellulose, methylcellulose, salts of polyacrylic acid,polyvinylpyrrolidone, gum arabic, and starch.

Ink Set

[0066] The ink set for ink jet recording according to the presentinvention comprises at least two ink compositions of the type describedabove, characterized in that

[0067] said ink set satisfies a relationship represented by formula (a)at least at 20° C.:

((μ max−μ min)/μ max)×100≦5 (%)  (a)

[0068] wherein μ max represents the maximum viscosity value in the inkcompositions of the ink set; and μ min represents the minimum viscosityvalue in the ink compositions of the ink set.

[0069] According to a preferred embodiment of the present invention, inthe ink set, the relationship represented by formula (a) is preferablyalways satisfied at 15 to 45° C. The actual service temperature of theink composition in ink jet printers is generally 15 to 45° C.

[0070] In the ink set according to the present invention, when two ormore ink compositions according to the present invention constitutingthe ink set satisfy the relationship represented by formula (a) at 20°C., the frequency characteristics of ink become preferred. Therefore, inprinting, the variation in dot size is reduced and, thus, printing canbe stably carried out.

[0071] In the ink set according to the present invention, the ink setcan be regulated so as to satisfy the relationship represented byformula (a) by properly regulating the amount of the compoundrepresented by formula (I) added in each of the ink compositionsconstituting the ink set.

[0072] When the ink compositions constituting the ink set are differentfrom each other or one another in ingredients, this means that themolecular weight of the ingredients and the particle diameter vary fromink to ink. Therefore, the viscosity of the whole ink set should beregulated. To this end, it is common practice to add additives such asglycerin. When ink compositions are greatly different from each other inmolecular weight or polarity of materials, the ink compositions aresometimes greatly different from each other in frequencycharacteristics. In this case, the amount of additives such as glycerinshould be increased to modify the viscosity of ink. Increasing theamount of the additive, however, is sometimes unfavorable because thisaffects other properties of the ink composition per se. The compoundrepresented by formula (I) is advantageous in that the amount of thecompound required for modifying the viscosity of the ink composition issmaller than that of glycerin required for this purpose. Therefore, theink set according to the present invention is advantageous in that theviscosity of each ink composition can be easily modified by properlyregulating the amount of the compound represented by formula (I) added.

[0073] In the present invention, when the ink composition included inthe ink set further contains glycerin, the viscosity of the inkcomposition can be easily modified by regulating the total amount ofglycerin and the compound represented by formula (I). This is becauseglycerin and the compound represented by formula (I) can be intimatelymixed together and, in addition, both the compounds are less likely toaffect frequency characteristics of inks.

[0074] According to another preferred embodiment of the presentinvention, at least two ink compositions contained in the ink set areidentical to each other in color but different from each other in colordensity. Here “at least two ink compositions identical to each other incolor but different from each other in color density” refers to, forexample, a combination of a magenta ink composition having high colordensity with a magenta ink composition having low color density (lightmagenta). Therefore, for example, also for other color ink compositions,such as black, cyan, yellow, orange, and green ink compositions, when anink composition having low color density (light ink) is present, thesame matter as described above in connection with the magenta inkcompositions is true of each of these color ink compositions.

[0075] According to other preferred embodiment of the present invention,in the ink set, the colorant contained in each of the ink compositionsis a pigment and the viscosities of the ink compositions as measuredwith a rotating viscometer at a torque of 1 mN·m to 100 N·m satisfy therelationship represented by formula (a). Here Rheometer RIS-2manufactured by Rheometrix Corp. may be mentioned as a specific exampleof the rotating viscometer suitable for use in the measurement of theviscosity (measured at 20° C.).

[0076] When the torque of the rotating viscometer falls within the abovedefined range, if the measured viscosities of the ink compositionssatisfy the relationship represented by formula (a), then it becomeseasy to grasp the pseudoplastic flow properties of each ink. Therefore,a difference in frequency characteristics between inks in ink jetrecording can be minimized.

[0077] According to a preferred embodiment of the present invention, forat least one ink composition contained in the ink set according to thepresent invention, the content of the colorant is not less than 5% byweight. The colorant content of not less than 5% by weight isadvantageous in that, in ink jet recording, the color development ofrecords is excellent.

Recording Method

[0078] The ink set according to the present invention can be used in arecording method wherein ink compositions are deposited onto a recordingmedium to perform printing.

[0079] According to another aspect of the present invention, there isprovided an ink jet recording method comprising the steps of: ejectingdroplets of an ink composition in the ink set; and depositing thedroplets onto a recording medium to perform printing.

[0080] According to the present invention, there is also provided arecord produced by the recording method.

[0081] Further, according to the present invention, there is furtherprovided an ink jet recording apparatus using the ink set for ink jetrecording that can provide an ink jet system capable of realizing stableejection.

EXAMPLES

[0082] The present invention will be described in more detail withreference to the following examples, which should not be construed aslimiting the scope of the present invention.

Preparation of Ink Compositions

[0083] The following ingredients were mixed together to prepare inkcompositions 1 to 11.

[0084] In the following, all of pigments 1 to 4 are a surface treatedpigment which has a dispersing group on its surface and is dispersiblein an aqueous solvent without any dispersant. These surface treatedpigments were prepared by oxidizing the surface of carbon black having aparticle diameter of 10 to 300 nm to introduce dispersing groups, suchas carbonyl, carboxyl, hydroxyl, or sulfone groups. On the other hand,pigments 5 and 6 are not a surface treated pigment, but pigmentsprepared by dispersing pigment particles with the aid of a dispersant,more specifically pigments prepared by dispersing carbon black particleswith the aid of a styrene-acrylic acid random copolymer dispersant.

[0085] In the following description, the numeric value withinparentheses after the pigment represents the average particle diameterof the pigment particles which is in nm.

[0086] In the following description, water-soluble dyes 1 to 4 arerespectively the following dyes.

[0087] Water-soluble dye 1: Direct Black 154

[0088] Water-soluble dye 2: Direct Yellow 132

[0089] Water-soluble dye 3: Direct Blue 86

[0090] Water-soluble dye 4: Acid Red 52

[0091] Further, in the following ink compositions, water as the balancecontained 0.001 to 0.05% of benzotriazole for preventing the corrosionof ink jet recording head members and 0.01 to 0.03% of EDTA for reducingthe influence of metal ions contained in the ink system.

[0092] In the following description, the content of each ingredient inthe ink composition was expressed in terms of the weight percentage ofthe weight of each ingredient based on the weight of the whole inkcomposition.

[0093] In the following description, DEGmBE represents diethylene glycolmonobutyl ether, TEGmBE triethylene glycol monobutyl ether, PGmBEpropylene glycol monobutyl ether, DPGmBE dipropylene glycol monobutylether, and DEGmME diethylene glycol monomethyl ether. Ink composition 1Amount added (wt %) Pigment 1 (105) 5.0 Compound 1 of formula (I) 8.0DEGmBE 8.0 OLFINE STG 0.8 Diethylene glycol 10.0  2-Pyrrolidone 5.0Triethanolamine 0.8 Ion-exchanged water Balance

[0094] Compound 1 is representated by formula (I) in whichm1+m2+m3+n1+n2+n3 is 2 and, at the same time, n1, n2, and n3 each are 0(zero). That is, Compound 1 has only ethyleneoxy (EO) in the parenthesesin formula (I). Ink composition 2 Pigment 2 (85) 4.5 Compound 2 offormula (I) 10.0  DPGmBE 3.0 DEGmBE 5.0 OLFINE E 1010 1.0 Dipropyleneglycol 5.0 Surfynol 465 1.2 Triethanolamine 0.9 Ion-exchanged waterBalance

[0095] Compound 2 is represented by formula (I) in whichm1+m2+m3+n1+n2+n3 is 0.8 and, at the same time, n1, n2, and n3 each are0 (zero). That is, Compound 2 has only ethyleneoxy (EO) in theparentheses in formula (I). Ink composition 3 Pigment 3 (90) 5.5Compound 3 of formula (I) 2.0 PGmBE 3.5 TEGmBE 5.0 Surfynol 104 0.3Diethylene glycol 7.0 Thiodiglycol 3.5 1,6-Hexanediol 5.0Diethylethanolamine 1.0 Potassium hydroxide 0.1 Ion-exchanged waterBalance

[0096] Compound 3 is represented by formula (I) in whichm1+m2+m3+n1+n2+n3 is 4 and, at the same time, ethyleneoxy andpropyleneoxy are present in a ratio of 3:1 in the parentheses in formula(I). Ink composition 4; Pigment 4 (80) 5.0 Water-soluble dye 1 1.0Compound 4 of formula (I) 8.0 DEGmBE 5.0 TEGmBE 5.0 Diethylene glycol3.0 1,5-Pentanediol 2.0 Dimethyl-2-imidazolidinone 2.0 Sodium benzoate0.1 Triethanolamine 0.7 Ion-exchanged water Balance

[0097] Compound 4 is represented by formula (I) in whichm1+m2+m3+n1+n2+n3 is 1.5 and, at the same time, n1, n2, and n3 each are0 (zero). That is, Compound 4 has only ethyleneoxy (EO) in theparentheses in formula (I). Ink composition 5 Pigment 1 (105) 3.0Water-soluble dye 1 1.0 Compound 5 of formula (I) 10.0  DEGmBE 9.0OLFINE STG 0.3 Glycerine 7.0 Triethanolamine 0.9 Ion-exchanged waterBalance

[0098] Compound 5 is represented by formula (I) in whichm1+m2+m3+n1+n2+n3 is 2.8 and, at the same time, n1, n2, and n3 each are0 (zero). That is, Compound 5 has only ethyleneoxy (EO) in theparentheses in formula (I). Ink composition 6 Water-soluble dye 2 5.0Compound 6 of formula (I) 6.0 DPGmBE 4.0 DEGmBE 8.0 Glycerin 10.0 Thiodiglycol 2.0 1,5-Pentanediol 1.0 Triethanolamine 0.9 Ion-exchangedwater Balance

[0099] Compound 6 is represented by formula (I) in m1+m2+m3+n1+n2+n3 is3.5 and, at the same time, n1, n2, and n3 each are 0 (zero). That is,Compound 6 has only ethyleneoxy (EO) in the parentheses in formula (I).Ink composition 7 Water-soluble dye 3 5.0 Compound 7 of formula (I)10.0  DEGmBE 8.0 Glycerin 5.0 Trimethylolpropane 1.0 Trimethylolethane1.0 Surfynol 465 1.0 Triethanolamine 0.5 KOH  0.05 Ion-exchanged waterBalance

[0100] Compound 7 is represented by formula (I) in whichm1+m2+m3+n1+n2+n3 is 10 and, at the same time, n1, n2, and n3 each are 0(zero). That is, in the case of Compound 7 has only ethyleneoxy (EO) inthe parentheses in formula (I). Ink composition 8 Water-soluble dye 45.5 Compound 8 of formula (I) 6.0 DEGmBE 5.0 Glycerin 5.0 Diethyleneglycol 5.0 Tetrapropylene glycol 5.0 Triethanolamine 0.9 KOH 0.1Ion-exchanged water Balance

[0101] Compound 8 is represented by formula (I) in whichm1+m2+m3+n1+n2+n3 is 4.5 and, at the same time, n1, n2, and n3 each are0 (zero). That is, Compound 8 has only ethyleneoxy (EO) in theparentheses in formula (I). Ink composition 9 (comparative) Pigment 5(90) 5.0 Glycerin 10.0  Dispersant 3.0 Nonionic surfactant 1.0Ion-exchanged water Balance Ink composition 10 (comparative)Water-soluble dye (Food Black 2) 5.5 DEGmME 7.0 Diethylene glycol 10.0 2-Pyrrolidone 5.0 Ion-exchanged water Balance Ink composition 11(comparative) Pigment 6 (110) 5.5 Water-soluble dye (Food Black 2) 2.5Diethylene glycol 10.0  Nonionic surfactant 1.0 Ion-exchanged waterBalance

Evaluation Test

[0102] Evaluation Test A: Ejection Stability

[0103] For all the ink compositions 1 to 8 thus prepared, the ejectionstability was good, and, according to evaluation criteria describedbelow, to which grade “A” was asigned.

[0104] Based on the ink compositions 1 to 8, groups of ink compositionsof Examples 1 to 8 were prepared in the following manner, and weretested for ejection stability. with regard to ink composition 1, thehalf of the colorant added in this composition was replaced with any oneof (1) the compound represented by formula (I) in the ink composition,(2) glycerin, (3) polyethylene glycol 1000, (4) ethylene oxide (30 mol)adduct of glycerin, and (5) polysaccharide [a mixture of tetra- orhigher straight-chain oligosaccharides (tradename: TETRUP H,manufactured by HAYASHIBARA SHOJI, Inc.)] to give prepare five inkcompositions 1-1 to 1-5. The viscosity was regulated so that thedifference in viscosity between the ink compositions thus prepared andthe ink composition 1 was not more than 5% based on the viscosity of theink composition 1. That is, the prepared ink compositions were regulatedso that the relationship represented by formula (a) was satisfiedbetween the prepared ink compositions and the ink composition 1. A groupof ink compositions based on ink composition 1 are designated as the inkcompositions of Example 1.

[0105] Each of the ink compositions 1-1 to 1-5 of Example 1 was testedfor ejection stability as follows. In the test, the ink composition tobe evaluated was loaded into an ink jet printer MJ-900C manufactured bySeiko Epson Corp., and printing was continuously carried out at 40° C.on 20000 pages of plain paper of size A4 (Xerox 4024, manufactured byXerox Corp.) (average number of printed characters per page: 1000).

[0106] The results were evaluated according to the following evaluationcriteria.

[0107] A: Dropouts of dots took place.

[0108] B: Dropouts of dots took place in 1 to 5 positions on average per100 pages.

[0109] C: Dropouts of dots took place in 6 to 20 positions on averageper 100 pages.

[0110] D: Dropouts of dots took place in not less than 21 positions onaverage per 100 pages.

[0111] In the same manner as described above, for each of inkcompositions 2 to 8, ink compositions, wherein the half of the colorantwas replaced with the materials (1) to (5) above, were prepared. Thus,groups of ink compositions respectively according to Examples 2 to 8were prepared. The ink compositions thus obtained were tested forejection stability in the same manner as described above.

[0112] The results were as summarized in Table 1.

Evaluation Test B: Print Quality

[0113] The ink compositions 1 to 11 thus obtained were provided, andwere then bidirectionally printed as a plurality of character linepatterns having different sizes on the following predetermined testpapers. The prints thus obtained were visually inspected for blurring ofcharacters and feathering of characters due to ink migration down paperfibers to evaluate print quality.

[0114] The printing was performed by means of an ink jet printer MJ-930Cunder conditions of 360 dpi and fine mode for plain paper. In thisevaluation test, plain papers commercially available in Europe, America,and Japan, specifically Conqueror (manufactured by Arjo Wiggins),Favorit (manufactured by XEROGRAFIE), Modo Copy (manufactured by Modo),Rapid Copy (manufactured by igepa), EPSON EPP (manufactured by SeikoEpson Corporation), Xerox P (manufactured by Xerox Corp.), Xerox 4024(manufactured by Xerox Corp.), Xerox 10 (manufactured by Xerox Corp.),Neenha Bond (manufactured by Kimberly-Clark), Ricopy 6200 (manufacturedby Ricoh Co., Ltd.), Yamayuri (manufactured by Honshu Paper Co., Ltd.),and Xerox R (manufactured by Xerox Corp.), were used as test papers.

[0115] The results were evaluated according to the following evaluationcriteria.

[0116] A: A Japanese Kanji character “

” in a JIS Minchou form at a size of 3 mm square could be printedwithout overlapping.

[0117] B: A Japanese Kanji character “

” in a JIS Minchou form at a size of 4 mm square could be printedwithout overlapping.

[0118] C: Overlapping was observed in a printed Japanese Kanji character“

” in a JIS Minchou form at a size of 4 mm square.

[0119] D: Overlapping was observed in a printed Japanese Kanji character“

” in a JIS Minchou form at a size of 5 mm square.

[0120] The results were as summarized in Table 2.

Evaluation Test C: Anti-cloging Property

[0121] The ink composition 1 was loaded into an ink jet printer MJ-930Cmanufactured by Seiko Epson Corporation, and the printer was allowed tostand under conditions of temperature 60° C. and relative humidity 40%for one week. One week after the initiation of the standing, thisprinter required not more than three cleaning operations for return tonormal ejection of the ink composition through all the nozzles.

[0122] On the other hand, the same test was carried out for the inkcompositions to which the compound represented by formula (I) had notbeen added. As a result, three or more cleaning operations werenecessary for return to normal ejection of the ink composition throughall the nozzles. The printer MJ-930C is a printer wherein a head using apiezoelectric element, which is an electrostrictive element, has beenadopted.

Evaluation Test D: Print Quality

[0123] With regard to ink composition 1, only carbon black as thecolorant was replaced by other pigments to prepare ink compositions ofother colors, i.e., Cyan, magenta, and yellow. Pigment Blue 15:3 wasused as cyan, Pigment Red 122 as magenta, and Pigment Yellow 138 asyellow. These pigments were dispersed in the ink compositions with theaid of a styrene-acryl dispersant. For the particle diameter of thepigments, cyan was 80 nm, magenta 100 nm, and yellow 95 nm. In thepreparation of the other color ink compositions, the compoundrepresented by formula (I) was added so that, when the viscosity (20°C.) of the ink composition 1 using carbon black was presumed to be 1,the other color ink compositions had respective predetermined viscosityvalues (relative ratio).

[0124] Thus, an ink set 1 comprising black, cyan, magenta, and yellowink compositions based on the chemical composition of the inkcomposition 1 was prepared.

[0125] The ink set 1 was loaded into an ink jet printer MJ-930Cmanufactured by Seiko Epson Corporation, and predetermined color patchesand 4000 characters per page were microdot printed by 500 pages of plainpaper of size A4. In this case, the prints were inspected for printdisorder, and the results were evaluated according to the followingevaluation criteria. The color patch pattern printed for the evaluationwas composed of 30% of blotted images, 10% of longitudinal lines, 10% ofhorizontal lines, 30% of characters (alphabetical characters andJapanese characters), and 20% of pictures.

[0126] A: Print disorder did not take place at all.

[0127] B: Print disorder took place 1 to 5 times.

[0128] C: Print disorder took place 6 to 10 times.

[0129] D: Print disorder took place 11 times or more.

[0130] The results were as summarized in Table 3. TABLE 1 Results ofevaluation test A (evaluation of ejection stability) Materialsubstituted for half Example amount of colorant 1 2 3 4 5 6 7 8 (1)Compound of formula (I) A A A A A A A A (2) Glycerin A B A A A A A A (3)Polyethylene glycol 1000 D C D D B C B C (4) Ethylene oxide (30 mol) D DD D C D C D adduct of glycerin (5) Polysaccharide (mixture of D D D D CD C D tetra- or higher straight-chain oligosaccharides)

[0131] TABLE 2 Results of evaluation test B (Print quality evaluationtest) Ink composition Type of paper 1 2 3 4 5 6 7 8 9 10 11 Conqueror AA A A A A A A C C C Favorit A A A A A A A A D D D Modo Copy A A A A A AA A C D D Rapid Copy A A A A A A A A C D D EPSON KPP A A A A A A A A C CD Xerox P A A A A A A A A B D D Xerox 4024 A A A A A A A A C D D Xerox10 A A A A A A A A C D D Neenha Bond A B A A A A A A C D D Ricopy 6200 AA A A A A A A C C D Yamayuri A B A A A A A A D D D Xerox R A A A A A A AA C D D

[0132] TABLE 3 Results of evaluation test D Ratio of viscosity of eachink composition to viscosity of black ink Black 1   1   1   1   1   1  1   Cyan 1.10 1.06 0.94 1.05 0.95 1.04 0.96 Magenta 1.10 1.06 0.94 1.050.95 1.04 0.96 Yellow 1.10 1.06 0.94 1.05 0.95 1.04 0.96 Evaluation D CC B B A A

1. An ink composition for ink jet recording, comprising a compoundrepresented by formula (I)

wherein EO represents an ethyleneoxy group; PO represents a propyleneoxygroup; m1, m2, m3, n1, n2, and n3 each are independently 0 (zero) or anatural number of not less than 1; EO and PO may be arranged, regardlessof order in the parentheses [ ], randomly or as blocks joined together;and m1+m2+m3+n1+n2+n3 is in the range of 0.5 to 10 in terms of numberaverage of a mixture of compounds represented by formula (I), and acolorant, provided that when, in formula (i), all of n1, n2, and n3 arezero, said ink composition further comprises glycerin.
 2. The inkcomposition according to claim 1, which, when none of n1, n2, and n3 informula (I) are zero, further comprises glycerin.
 3. The ink compositionaccording to claim 1 or 2, wherein the colorant comprises awater-soluble dye and/or a pigment.
 4. The ink composition according toclaim 3, wherein the pigment is a surface treated pigment which has adispersing group on its surface and is dispersible in an aqueous solventwithout any dispersant.
 5. The ink composition according to any one ofclaims 1 to 4, wherein the compound represented by formula (I) is amixture of at least two or more compounds selected from the groupconsisting of the compounds represented by formula (I).
 6. The inkcomposition according to any one of claims 1 to 5, wherein the compoundrepresented by formula (I) has an average molecular weight of not morethan
 1000. 7. The ink composition according to any one of claims 1 to 6,wherein the ink composition has a surface tension of not more than 40mN/m.
 8. The ink composition according to claim 7, which furthercomprises a 1,2-alkylene glycol in which the alkylene group may bebranched.
 9. The ink composition according to claim 8, wherein, when theamount of 1,2-alkylene glycol added is not less than 3% by weight, theamount of the compound represented by formula (I) added is at least 2%by weight.
 10. The ink composition according to claim 9, wherein thealkylene group of 1,2-alkylene glycol has 4 to 10 carbon atoms.
 11. Theink composition according to claim 10, wherein the 1,2-alkylene glycolis 0.5 to 20% by weight of 1,2-pentanediol, 0.3 to 15% by weight of1,2-hexanediol, or a mixture of said 1,2-alkylene glycols.
 12. The inkcomposition according to any one of claims 8 to 11, which furthercomprises 0 to 10% by weight of (di)propylene glycol monobutyl ether andthe weight ratio of the 1,2-alkylene glycol to (di)propylene glycolmonobutyl ether is 1:0 to 1:10.
 13. The ink composition according to anyone of claims 8 to 11, which further comprises 0 to 5% by weight of anacetylene glycol surfactant and the weight ratio of the 1,2-alkyleneglycol to the acetylene glycol surfactant is 1:0 to 1:3.
 14. The inkcomposition according to any one of claims 8 to 11, which furthercomprises 0 to 20% by weight of di(tri)ethylene glycol monobutyl etherand the weight ratio of the 1,2-alkylene glycol to the di(tri)ethyleneglycol monobutyl ether is 1:0 to 1:10.
 15. An ink set for ink jetrecording, comprising at least two ink compositions according to any oneof claims 1 to 14, characterized in that said ink set satisfies arelationship represented by formula (a) at least at 20° C.: ((μ max−μmin)/μ max)×100≦5 (%)tm (a) wherein μ max represents the maximumviscosity value in the ink compositions contained in the ink set; and μmin represents the minimum viscosity value in the ink compositionscontained in the ink set.
 16. The ink set according to claim 15, whichalways satisfies the relationship represented by formula (a) at 15 to45° C.
 17. The ink set according to claim 15 or 16, wherein said atleast two ink compositions are identical to each other in color butdifferent from each other in color density.
 18. The ink set according toany one of claims 15 to 17, wherein the colorant contained in each ofthe ink compositions is a pigment and the viscosities of the inkcompositions as measured with a rotating viscometer at a torque of 1mN·m to 100 mN·m satisfy the relationship represented by formula (a).19. The ink set according to any one of claims 15 to 18, wherein atleast one of the ink compositions has a colorant content of not lessthan 5% by weight.
 20. An ink jet recording method comprising the stepsof: ejecting droplets of an ink composition; and depositing the dropletsonto a recording medium to perform printing, the ink composition beingan ink composition in the ink set according to any one of claims 15 to19.
 21. A record produced by the method according to claim 20.